So how many negations do you want here? There are three concepts that can be negated here: (1) pleasing appearance (2) concealment (3) little worth. If you negate all three of them you get an unpleasant appearance displaying something of great value. If you just negate (1) and (3) you get an unpleasant appearance concealing something of great worth, which is maybe tarnished, though that doesn't guarantee the worth. Or you could negate only (1) and (2) and have an unpleasant appearance displaying something of little worth, which is junk. "Opposite" is not a simple concept.
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John LawlerMay 17 '13 at 15:15

A rough diamond is the nominal term; there is a tendency to form compound as well as simple nominal modifiers. A rough-diamond colleague.
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Edwin AshworthMay 17 '13 at 16:25

In the "for what it's worth" department: The late country music singer/songwriter Johnny Cash had a song that went: "I'm just an old chunk of coal now, Lord, but I'm gonna be a diamond some day." What Cash is saying, I think, is that he's far from perfect now, but just as the combination of carbon + pressure + eons = a diamond, likewise he will, through a transformative process, emerge as a diamond one day. Or to change the metaphor, today on the outside he's a pauper, but inside is a prince waiting to come out.
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rhetoricianMay 17 '13 at 17:04

The story of "The Rich Man and Lazarus" is illustrative of the opposite of gilded (see Luke 16:19-31). On the one hand, you have a well-to-do member of the gentry, who from all outward appearances has everything he wants and needs. On the other hand you have a poor, pathetic beggar with ugly sores all over his body, who from all appearances has nothing to recommend him. Who goes to paradise? The rich man? No, Lazarus does. Which goes to show that just as surely as the proud will someday be abased, so too someday will the humble be exalted--albeit not necessarily in this lifetime!
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rhetoricianMay 17 '13 at 17:23

The word you are looking for is something like patina or tarnish. It should be a word that implies something that obscures the beauty, although as patina is a sign of proof of age, many do not see it as a negative.